Init levels

0

Go into boot prompt (OK).

1

Put the system in system administrator mode. All file systems are
mounted. Only a small set of essential kernel processes are left running.
This mode is for administrative tasks such as installing optional utility
packages. All files are accessible and no users are logged in on the
system.

2

Put the system in multi-user mode. All multi-user environment terminal
processes and daemons are spawned. This state is commonly referred to as
the multi-user state.

3

Start the remote file sharing processes and dae mons. Mount and
advertise remote resources. Run level 3 extends multi-user mode and is
known as the remote-file-sharing state.

4

Is available to be defined as an alternative multi-user environment
configuration. It is not necessary for system operation and is usually not
used.

5

Shut the machine down so that it is safe to remove the power. Have the
machine remove power, if possible.

6

Stop the operating system and reboot to the state defined by the
initdefault entry in /etc/inittab.